Coherent detection has been put into practice as a technique for realizing optical communications of a large capacity. A coherent optical receiver includes a local light source, and mixes a received optical signal and local oscillation light. The carrier wavelength of the received optical signal and the wavelength of the local oscillation light are roughly equal. The mixed light is converted into an electric signal by a photo detector. This electric signal is amplified by an amplifier (such as a transimpedance amplifier (TIA)), and is guided to a signal processing circuit. The signal processing circuit includes a demodulation circuit etc., and recovers a transmitted signal.
In the above coherent optical receiver, in order to increase the sensitivity of the photo detector, an input optical intensity (or an input optical power) for the photo detector needs be an appropriately controlled. Accordingly, a coherent optical receiver in many cases includes a variable optical attenuator (VOA) for adjusting the intensity of a received optical signal. In such a case, the coherent optical receiver has a function of detecting the intensity of the received optical signal. The variable optical attenuator is controlled based on detection results of this function such that the optical signal of preferable intensity is guided to the photo detector.
As described above, a coherent optical receiver has a function of detecting the intensity of a received optical signal. The intensity of a received optical signal is detected based on a gain of an amplifier that amplifies the optical signal (U.S. Patent Publication No. 2016/0164624 for example).
As another technique for realizing optical communications of a large capacity, polarization multiplexing has been put into practice. In polarization multiplexed communications, a pair of polarizations (X polarization and Y polarization) that are orthogonal to each other is used for transmitting data. Therefore, the transmission capacity per wavelength in polarization multiplexed communications is twice that of a communication that does not adopt polarization multiplexing.
When a coherent optical receiver receives a polarization multiplexed optical signal, an I-component and a Q-component are detected for each polarization. Accordingly, electric signals respectively corresponding to four lanes (XI, XQ, YI and YQ) are generated from a polarization multiplexed optical signal, and each electric signal is amplified by an amplifier. Then, a processor included in the coherent optical receiver obtains an input optical intensity of each lane based on a gain of each amplifier so as to calculate the sum of the intensities, and thereby detects the input optical intensity of the polarization multiplexed optical signal. This increases loads on the processor for calculating an input optical intensity when a coherent optical receiver receives a polarization multiplexed optical signal.
Note that, in order to downsize a coherent optical receiver, it is desirable that the processor execute a plurality of tasks. This leads to a risk that when heavy loads for calculating an input optical intensity are imposed on a processor, other tasks may be delayed.